A storage phosphor sheet retains a stored image when exposed to an image-wise pattern of ultra short wavelength radiation, such as X-rays. The stored latent image may then be read by illuminating the phosphor sheet with radiation of a relatively long stimulation wavelength, typically red or infrared light. Upon stimulation, the phosphor emits radiation in a shorter emission wavelength band separated from the stimulating wavelength band, typically green, blue, or ultraviolet light, with an emitted intensity proportional to the quantity of ultra short wavelength radiation to which the phosphor had been originally exposed when the latent image was recorded. In a typical phosphor imaging system, the phosphor sheet storing a latent image is stimulated with a scanning beam, such as a laser beam, at the stimulation wavelength illuminating successive spots on the phosphor sheet. The emitted radiation from an illuminated spot of the phosphor sheet is collected by an optical collector and sensed by a photodetector, such as a photomultiplier tube, to produce electronic image signals corresponding to the intensity of the emitted radiation.
One relatively simple imaging system is described by Matsuda et al. in U.S. Pat. No. 4,680,473. Radiation emitted from the phosphor sheet enters a light guide member through an input face positioned close to the scan line on the phosphor and is directed by the light guiding member to a photomultiplier. In order to improve collection efficiency, a mirror is positioned to specularly reflect more of the emitted radiation to the guide member's input face. In one embodiment, the mirror has a specular reflection surface that selectively reflects radiation at the phosphor's emission wavelength, but either transmits through the mirror or absorbs (and in any case, does not reflect) radiation at the stimulating wavelength. In another embodiment, a filter plate, which transmits the emission wavelength but absorbs the stimulating wavelength, is placed between the scan line and the mirror. Radiation emitted from the phosphor is directed toward the mirror and passes once through the filter plate on its way to the mirror and once more on its way from the mirror to the guide member. In either embodiment, the input face of the light guiding member may have an antireflection surface coating that minimizes or eliminates reflection of both the emission and stimulating wavelengths, while a filter at the back of the light guiding member blocks the stimulating wavelength from being received by the photomultiplier. By reducing the reflection of the stimulating wavelength from the mirror or the light guiding member's input face, the stimulating radiation can be prevented from re-illuminating the phosphor in areas outside of the scan spot.
It is desirable to collect as much of the emitted radiation as possible and to direct it to the photodetector with little, if any, positional variation in the amount of radiation collected, in order to obtain a strong signal. With that objective in mind, other more efficient collectors have been developed. One such collector is described by Hideo Noda in the Japanese Patent Application Laying-Open No. 6-160311, published Jun. 7, 1994. The collector includes a set of fluorescence-reflecting flat mirror plates essentially forming the sides of an open box, with the open side adjacent to the recorded surface of a phosphor imaging plate and the opposite side containing a photomultiplier tube receiving emitted fluorescence through a noise removing filter that transmits only the fluorescence wavelength (about 390 nm). One of the fluorescence reflecting sides is a dichroic mirror that transmits excitation radiation (a 633 nm wavelength laser beam) from an optical source. Diagonally disposed inside the collector is another dichroic mirror that reflects the excitation beam from the laser source and directs it to the recorded surface of the phosphor plate, but transmits fluorescence emitted from phosphor to the photomultiplier tube.
While such a configuration is adapted to collect the phosphor's fluorescence emission for detection by a photomultiplier tube, it inherently allows re-illumination by stimulating radiation in regions of the phosphor outside of the spot illuminated by the excitation laser beam. This is because some of that excitation light is not absorbed by the phosphor plate but reflects or scatters therefrom and is collected along with the fluorescence. Although the excitation is blocked by the diagonal dichroic mirror and the noise rejecting filter from reaching the photomultiplier tube, the mirror sides of the collector together with the diagonal dichroic mirror reflect radiation of the stimulating wavelength so that some of it can re-illuminate the phosphor.
One solution to this problem is provided in the collector described in U.S. Pat. No. 5,598,008 to Livoni. That patent teaches a hollow cylindrical, ellipsoidal, or spherical collector with a wavelength selective diffusely reflective interior surface coating. The diffusely reflecting interior coating helps to smooth out the signal variations that are a result of the position of the stimulating radiation beam along the scan path. The coating is a barium sulfate based material that is doped so as to absorb radiation of the stimulating wavelength, while still highly reflecting radiation in a phosphor's emission wavelength band. Unfortunately, diffuse reflection coatings with the required high reflectance (near 98%) at the emission wavelengths that need to be collected, lack sufficiently low reflectance at the stimulating wavelength to sufficiently minimize re-illumination. Coating materials simply do not provide the desired reflectance range. At best, a reduction to about 80% reflectivity at the stimulation wavelength is possible. Lower reflectances require reducing the reflectance at the emission wavelengths as well, reducing the amount of light collected.
Accordingly, it is an object of the invention to minimize re-illumination at the stimulating wavelength in diffusely reflective integrating collectors, by providing an optical radiation collector arrangement with much lower reflectance at the stimulating wavelength than can be provided by known wavelength selective surface coatings.